The present invention relates to a color picture tube having an improved inline gun, and particularly to an improvement in the electron gun for providing a nonlinear correction in raster sizes (also called nonlinear coma correction) within the tube.
An inline electron gun is one designed to generate preferably three electron beams in a common plane and direct those beams along convergent paths in that plane to a point or small area of convergence near the tube screen.
A problem that exists in a color picture tube having an inline gun is a coma distortion wherein the sizes of the three rasters scanned by the three beams on the screen by an external magnetic deflection yoke are different because of the eccentricity of the two outer beams with respect to the center of the yoke.
Messineo et al. U.S. Pat. No. 3,164,737, issued Jan. 5, 1965, teaches that a similar coma distortion caused by using different beam velocities can be corrected by use of a magnetic shield around the path of one or more beams in a three gun assembly. Barkow U.S. Pat. No. 3,196,305, issued July 20, 1965, teaches the use of magnetic enhancers adjacent to the path of one or more beams in a delta gun, for the same purpose. Krackhardt et al. U.S. Pat. No. 3,534,208, issued Oct. 13, 1970, teaches the use of a magnetic shield around the middle one of three inline beams for coma correction. Yoshida et al. U.S. Pat. No. 3,548,249, issued Dec. 15, 1970, teaches the use of C-shaped elements positioned between the center and outer beams to enhance the effect of the vertical deflection field on the center beam. Murata et al. U.S. Pat. No. 3,594,600, issued July 20, 1971, teaches the use of C-shaped shields around the outer beams with the open sides of the members facing each other. These shields appear to shunt the vertical deflection field around all three beams. Takenaka et al. U.S. Pat. No. 3,860,850, issued Jan. 14, 1975, teaches the use of V-shaped enhancement members located above and below three inline beams and the use of C-shaped shields around the two outer beams. Hughes U.S. Pat. No. 3,873,879, issued Mar. 25, 1975, teaches the use of small disc-shaped enhancement elements above and below the center beam and ring shaped shunts around the two outer beams.
It has been found that for some tubes, the coma height grows nonlinearly with beam deflection on the screen. All of the foregoing coma correction devices only correct for linearly growing coma. Therefore, there is a need for coma correction members that will provide a nonlinear correction for nonlinearly growing coma.